In order to reduce the fuel consumption of the engine, modern vehicles have an idle-stop function that cuts in when the vehicle stops at traffic lights and the like, so that the engine stops frequently during town driving. The temperature of the engine lubricating oil therefore does not rise sufficiently during short trips to the shops and so on, and the trip is over before water mixed up in the oil can evaporate and be expelled. With PHV (Plug-in-Hybrid) vehicles and the like too, the engine similarly will have failed to reach a sufficient temperature when the vehicle stops after short commuting or shopping trips due to the on-off switching of engine revolutions as required. Water vapour created by combustion of the fuel therefore enters the sump together with blow-by gas, and because the engine is not hot enough, it condenses in the sump to form water droplets and these become mixed into the engine lubricating oil.
Furthermore, renewable biofuels have increasingly been used in automotive gasoline and light oils in recent years from the standpoint of reducing carbon dioxide emissions to counter global warming.
For example, plans are being pursued under the Japanese Energy Supply and Security Act for year-on-year reductions in greenhouse gases (CO2) by incorporating such renewable biofuels into automotive gasoline. In fact, 210,000 KL/year of biofuel, as the crude oil equivalent, was used in automotive gasoline in 2010, and it is planned that 500,000 KL/year of biofuel, as the crude oil equivalent, should be used by 2017.
These biofuels, specifically bioethanol or bioETBE (ethyl tert-butyl ether), are fuels for internal combustion engines containing high proportions of hydrogen (H/C) even among the hydrocarbons used in fuels, and so generate more water (water vapour) associated with combustion than ordinary fuels. The H/C (hydrogen/carbon) ratio of commercial premium gasoline and regular gasoline is respectively 1.763 and 1.875 calculated from the carbon concentrations shown in Table 2.4-1 of Oil Industry Promotion Center: 2005 Automotive Fuel Research Findings Report PEC-2005JC-16, 2-14. If 3% of such premium gasoline and regular gasoline were to be replaced with (bio)ethanol or similar, their H/C ratios would be respectively about 1.80 and 1.91. H/C thus rises as a result of using biofuel in gasoline, and although there is less carbon dioxide due to combustion, more water vapour is generated. Similarly, looking at the H/C ratios for commercial light oils, ‘BASE’ corresponding to a commercial light oil 2 in Table 4.1.1-2 of Oil Industry Promotion Center: 2008 Research and Development Findings Report on Diversification and Efficient Use of Automotive Fuels 14 has H/C of 1.91, and JIS2 diesel light oil has H/C of 1.927 according to Table 2 of Traffic Safety Environment Laboratory, Forum 2011 Data, “Adopting the trends and traffic research on advanced automotive fuels in the International Energy Agency (IEA)”. If 5% of these were replaced with methyl stearate as a typical biodiesel, H/C would rise to about 1.93 and although less carbon dioxide would be generated by combustion, on the other hand, more water vapour would be produced.
The situation is similar for the engines of vehicles that run on fuels of natural gas, LPG or propane, which have high hydrogen-carbon (H/C) ratios.
The most recent petrol engine oil standards, API-SN+RC (Resource Conserving) and ILSAC GF-5 standards, require that even vehicles using E85 fuels containing bioethanol should have the capacity to ensure that any (condensed) water or E85 fuel is emulsified and incorporated within the engine oil, so that any water from combustion and unburnt ethanol become mixed with the engine oil and water droplets will not precipitate out on metal surfaces to cause rust or corrosion around them (ASTM D7563: Emulsion Retention). Emulsion retention (emulsion stability) is a test with evaluation procedures laid down in ASTM D7563. This is a test to check and evaluate the stability of engine oil in respect of whether any (condensed) water or E85 fuel and the like that has become mixed with it does not deposit out on surfaces but remains incorporated in emulsion form without separating out, so that the individual engine components do not rust or corrode.
Furthermore, in recent years, ashless friction modifiers such as fatty acid esters have come to be added to engine lubricating oils so as to reduce friction between metals in the engine and improve fuel economy (Laid-open Patent JP2004-155881A; Tribologist, Namiki N, Vol. 48, 11 (2003), 903-909).
Organic molybdenum compounds and the like are often used as friction modifiers. However, ashless friction modifiers (i.e. leaving no ash residue when combusted as they contain no elements such as metals or phosphorus) that do not harm exhaust gas treatment equipment such as exhaust gas catalysts or diesel particulate filters (DPF) and do not affect the environment either have been preferred in recent years.
As such ashless friction modifiers added to engine lubricating oils contain neither metals nor elements such as phosphorus, they are known to have little effect on exhaust gas catalysts or exhaust gas post-treatment systems, and to be readily usable in engine lubricating oils. On the downside, they have a surfactant effect and, in some cases, this may intensify anti-emulsifying properties or water separability in the engine oil and cause water to deposit out on surfaces more readily. It has been feared that the deposited water would induce rusting or corrosion by coming into contact with the individual parts in the engine.
In particular, monoglyceride ashless friction modifiers are known to be highly effective for reducing friction and to be suitable for engine lubricating oil compositions, but if condensed water from water vapour associated with fuel combustion in the engine gets into the engine oil as described previously, it has been feared that this would increase anti-emulsifying properties or water separability.
Lubricating oil compositions for internal combustion engines that not only provide outstanding wear resistance and fuel economy (low-friction characteristics) but also cause condensed water from water vapour produced by fuel combustion to be dispersed through the oil to prevent corrosion or rusting of the engine have been being sought for this reason.
The present invention was devised in the light of the above situation and seeks to provide a lubricating oil composition for internal combustion engines that, as well as providing outstanding wear resistance and fuel economy, causes condensed water etc. from water vapour produced as a result of fuel combustion to be dispersed in the oil, so preventing corrosion or rusting of the engine.
On checking the anti-emulsifying properties and water separability of the monoglycerides with a specific structure used as ashless friction modifiers in specific engine lubricating oils {in particular, at least one base oil selected from the group consisting of base oils of Groups 2, 3 and 4 in the API (American Petroleum Institute) base oil categories with kinematic viscosity of from 3 to 12 mm2/s at 100° C. and viscosity index of not less than 100}, the present inventors established that when condensed water from water vapour associated with fuel combustion in the engine becomes mixed in with the engine oil, monoglycerides with the said specific structure increase anti-emulsifying properties or water separability in connection with the aforesaid specific engine lubricating oils and make separation of the water onto surfaces more prone to occur. They therefore established that using monoglycerides with the said specific structure on their own serves to reduce resistance to rusting or corrosion, and that the aforesaid specific engine lubricating oil compositions containing monoglycerides with the said specific structure do not comply with the most recent petrol engine oil standards API-SN+RC and ILSAC GF-5.
The present inventors further undertook wide-ranging studies and research on ways of improving emulsion stability in the aforesaid specific engine lubricating oils. They discovered that if a base oil mixture comprising at least two base oils in different API (American Petroleum Institute) categories was used together with the aforesaid monoglyceride ashless friction modifiers with a specific structure, and the properties of the aforesaid base oil mixture (sulphur content present in the base oil mixture and % CA in the base oil mixture, etc.) were set to within specific ranges, the lubricating oils showed improved emulsion stability in addition to outstanding wear resistance and fuel economy. They thus perfected the present invention.